[InstCombine] Use less bitwise operations to handle Instruction::SExt in...

[InstCombine] Use less bitwise operations to handle Instruction::SExt in SimplifyDemandedUseBits. Other improvements.

The current code created a NewBits mask and used it as a mask several times. One of them just before a call to trunc making it unnecessary. A call to getActiveBits can get us the same information for the case. We also ORed with this mask later when we should have just sign extended the known bits.

We also called trunc on the guaranteed to be zero KnownZeros/Ones masks entering this code. Creating appropriately sized temporary APInts is probably better.

Differential Revision: https://reviews.llvm.org/D32098

llvm-svn: 303779